Where machines subjected to high thermal and mechanical load are concerned, such as for example, compressors, gas turbines or steam turbines, it is desirable to reduce mechanical stresses by means of a suitable design of the individual machine and plant parts.
Thus, from the prior art, it is known, for example (see EP-A1-0 945 594 or U.S. Pat. No. 6,478,539 B1), in the moving blades of gas turbines, to design the transition from the blade leaf to the adjoining blade platform lying beneath it with a predetermined, preferably elliptic curvature contour, the major axis running in the radial direction and the minor axis being oriented parallel to the surface of the platform.
Furthermore, it is known from U.S. Pat. No. 6,237,558 B1 to provide specific locations of the crankcase of an internal combustion engine which are critical in terms of mechanical stresses with a curvature which follows a conic section (ellipse, hyperbola, parabola).
Not only the moving blades of turbines are exposed to high mechanical loads on account of the high rotational speeds, but also the rotor shaft itself. Critical locations are in this case, above all, the grooves in the rotor shaft which are arranged on the outer circumference and which, running in the axial direction or running around annularly, may be provided, for example, for receiving the blade roots of the moving blades or as part of a shaft seal. Where such grooves are concerned, the stresses arising in the groove depend critically on the cross-sectional contour. GB-A-2 265 671 or U.S. Pat. No. 4,818,182 discloses grooves running around annularly for the fastening of moving blades, said grooves having a rounded cross-sectional contour. No information is given on the nature of the curvature profile or on the influence of the contour on the stresses in the groove.
In the rotor parts subjected to particularly high thermal load, the turbine part, additional cooling measures are often provided, in order, at the high hot-gas temperatures, to achieve a sufficient service life of the material used. Cooling measures of this kind include cooling air ducts which run approximately in the radial direction from the inside outward through the rotor shaft and lead cooling air from an inner cooling air supply to the surface of the rotor shaft. Cooling air ducts of this type, however, constitute mechanical weakenings of the rotor shaft which may have an adverse effect in the case of the high temperatures and centrifugal forces and under the changing loads.